Animation of a Cilium

A cilium consists of a membrane-coated bundle of fibers called
an axoneme. An axoneme contains a ring of 9 double microtubules
surrounding two central single microtubules. Each outer doublet
consists of a ring of 13 filaments (subfiber A) fused to an assembly
of 10 filaments (subfiber B). The filaments of the microtubules
are composed of two proteins called alpha and beta tubulin. The
11 microtubules forming an axoneme are held together by three
types of connectors: subfibers A are joined to the central microtubules
by radial spokes; adjacent outer doublets are joined by linkers
that consist of a highly elastic protein called nexin; and the
central microtubules are joined by a connecting bridge. Finally,
every subfiber A bears two arms, an inner arm and an outer arm,
both containing the protein dynein.